Nithsdale is the valley of the River Nith in Dumfriesshire and Kirkcudbrightshire. In ancient days it was known as Ystrad Nidd, in the Old Welsh language which dominated these hills until the English language came, and the overlay of Irish Gaelic in the late Dark Ages.
The River Nith flows north to south through the Southern Uplands separating the Lowther Hills from the Scaur Hills, and runs mainly through Dumfriesshire. In its lower parts the river is the division between that county and Kirkcudbrightshire.
Nithsdale has historically been a strategic area as it forms an invasion route from England towards the north.